I have been to this buffet twice, once for their Sunday champagne brunch (inexpensive bottomless mimosas?? Yes!) and once for dinner.
First of all, I am in LOVE with The Cosmo; I went to the grand opening and it has one of my favorite bars (Vesper), not to mention it is gorgeous! The Wicked Spoon is my current favorite buffet as well-- the food is AMAZING. It is really hard not to over eat here because it all tastes so good and the layout is nice so that there really are no super crowded areas (at least there weren't any jam-ups while I was there). Wine is reasonable for the Strip ($20/bottle for the house cab) and I loved all the little mini serving dishes.
The service was standard for a Strip buffet (not the speediest) but our waitress was friendly and it IS a buffet....
If you're on the strip and debating whether or not to try out this place, DO IT! You and your belly will be happy you did!